WO2000079643A1

WO2000079643A1 - Radio communication base station antenna

Info

Publication number: WO2000079643A1
Application number: PCT/FR2000/001646
Authority: WO
Inventors: Thierry Lucidarme
Original assignee: Nortel Matra Cellular
Priority date: 1999-06-18
Filing date: 2000-06-14
Publication date: 2000-12-28
Also published as: EP1114488A1; CN1314013A; FR2795240B1; US6369774B1; CA2339875A1; FR2795240A1; BR0006874A; JP2003502975A

Abstract

The invention concerns an antenna comprising several primary sources (6A-6C) independently powered and arranged so as to exhibit different radiating characteristics. Said primary sources are placed in a first medium (7A-7C) so as to be spatially decoupled. A second medium (8A-8C) with a substantially lower impedance characteristic than the first medium, covers the first medium. Each primary source has a focusing direction (A-C) perpendicular to the interface between the first and second media, along which the distance (d1) between said primary source and said interface is μ1.(2p1)/4 and the second medium has a thickness (e2) equal to μ2.(2p2-1)/4, wherein μ1 and μ2 represent the wavelengths radiated by said primary source in the first and second media respectively, and p1 and p2 are integers.

Description

RADIOCOMMUNICATION BASE STATION ANTENNA

The present invention relates to antennas used in cellular radio base stations

The boom in cellular radiocommunications requires many sites to install base stations Cellular operators may find it difficult to find such sites In addition to the problems of site availability, there is the problem of the discomfort perceived by the public of due to the size and lack of aesthetics of the antennas of the base stations which, for the efficiency of the network, must of course be placed in height and in a visible way In certain countries, regulations, or charges, aim at limit the number of these antennas

The use of multisectoπelle antennas makes it possible to reduce the number of base station sites for a given coverage (see EP-A-0 802 579) However, these multisectoπelle antennas, because of their directivity and their multiplicity, are significantly more bulky than omni-directional antennas

To increase the directivity gain of a base station antenna, use is made of a network of radiating elements arranged in a particular manner with respect to the wavelength to be transmitted, and supplied by the same radio signals to which laws are applied. phase shift and appropriate amplitude The dimensions of the network are all the more important when seeking a high directivity gain The order of magnitude of the dimension of each radiating element is the transmitted wavelength, that is ie in the decimetric range, and their network arrangement leads to antennas whose dimensions can be from one to several meters

The difficulties mentioned above are further aggravated by the deployment of networks using different wavelength ranges. For example, in Europe, second generation digital systems use a band close to 900 MHz (GSM, "Global System for Mobile" communications ”) and a neighboring band of 1800 MHz (DCS,“ Digital Cellular System ”), and future third generation systems (UMTS,“ Universal Mobile Telecommunication System ”) will use a frequency band close to 2000 MHz To set up an infrastructure of a new type of network, an operator already operating another type of network must provide new antennas Either it has new installation sites, either it has to multiply the antennas on its preexisting sites In both cases, the antennas proliferate

In addition, the installation on the same site of antennas operating in frequency ranges whose ratio is a small integer poses insulation problems due to the reception by a harmonic antenna of the frequencies emitted by another antenna Ce scenario is that of the GSM and DCS bands, for which it is estimated that the antennas, already bulky, must be spaced at least 50 centimeters

A main aim of the present invention is to propose an arrangement of antennas which makes it possible to associate radiating elements having different radiation characteristics (directivity and / or frequency) in a relatively compact arrangement, in order to limit the difficulties mentioned above. -above

The invention thus provides a radiocommunication base station antenna, comprising several primary sources supplied independently and arranged so as to have different radiation characteristics, the primary sources being placed in a first medium so as to be spatially decoupled According to the invention, the antenna further comprises at least a second medium covering the first medium and having a characteristic impedance substantially lower than the first medium Each primary source has at least one focusing direction perpendicular to the interface between the first and second medium, according to which the distance d., between said primary source and said interface is substantially equal to λ-, (2p ₁ -1) / 4 and the second medium has a thickness e ₂ substantially equal to λ ₂ (2p ₂ - 1) / 4, where λ-, and λ ₂ denote the wavelengths radiated by said primary source in the first and second mili them, respectively, and p-, and p ₂ are integers

The media surrounding the primary sources present resonance conditions which provide a gain in directivity, in site and possibly in azimuth. The physical principle of this resonance has been described for the case of antennas conformed in the article "Gain Enhancement Methods for Pπnted Circuit Antennas "by DR Jackson et al, IEEE Transactions on Antennas and Propagation, Vol AP-33, No 9, September 1985, pages 976- 987 The gain in amplitude provided by the first and second media, of respective characteristic impedances Z _c1 and Z _c2 , is of the order of 2.Z _c1 / Z _c2 .

The characteristic impedance Z _c of a medium of relative dielectric permittivity ε _r and of relative magnetic permeability μ _r is given by

Z _c = Z _c0 with Z _c0 = 120π. As a result, the first and second

media can have parameters ε _r and μ _r adapted according to the desired gain.

In a preferred embodiment, we will essentially play on the dielectric permittivities ε _r , in order to use more readily available materials. In general, we will take a medium with strong ε _r for the second medium and ε _r ≈ 1 in the first medium in order to maximize the ratio

^ d ^ c2 = i ^μ1 ≈ J— (with ε _r = ε-,, μ _r = μ-, in the first medium and ε _r = ε _?, V ^ε ι -μ2 V ^ε ι μ _r = μ ₂ in the second medium).

Composite materials can also be used, which allows the values of ε _r and / or μ _r to be adjusted as required. In order to further increase the gain of the antenna, the first medium can be covered by a superposition of focusing layers, the first focusing layer, adjacent to the first medium, being formed by said second medium, and each focusing layer being formed by a medium of thickness substantially equal to λ _j . (2p _r 1) / 4 along the focusing direction of each of the primary sources, where λ _j denotes the wavelength radiated by said primary source in the medium forming said focusing layer and P _j is an integer. The i-th focusing layer is formed, for each odd integer i, by a medium having a characteristic impedance substantially lower than the media located on either side of this i-th focusing layer. The i-th focusing layer can in particular be formed, for each odd integer i, by a medium having an ε _r substantially higher than the media located on either side of this i-th focusing layer.

Increasing the number of focusing layers increases the gain in amplitude, which is of the order of 2. TT - - - - if there are 2k layers of _{m = 0} ^z c (2m) focusing over the central medium with high impedance, and of the order of k 7

2 d - ^{m ~} if there are 2k-1 focusing layers, Z _cj denoting for i> 2 _{m =} ι ^Z c (2m) the characteristic impedance of the (it) -th focusing layer (see HY Yang et al., "Gain Enhancement Methods for Printed Circuit Antennas through Multiple Supertrates", IEEE Transactions on Antennas and Propagation, Vol. AP-35, No. 7, July 1987, pages 860-863).

In one embodiment of the antenna according to the invention, the primary sources are supplied and arranged so as to radiate at different wavelengths. The antenna is then adapted to sites where base stations operating in different frequency bands are installed.

The dielectric media can be arranged parallel to a ground plane, the antenna then being able to be installed on a wall. In another advantageous arrangement, the primary sources are arranged along an axis around which said media have a symmetry of revolution. It is then possible to produce omnidirectional and / or multisectoral antennas having a reduced bulk.

Other particularities and advantages of the present invention will appear in the description below of nonlimiting exemplary embodiments, with reference to the appended drawings, in which:

- Figure 1 is a representation of a base station equipped with an antenna according to the invention;

- Figures 2 and 4 are perspective diagrams of an omnidirectional antenna and a trisectoral antenna according to the invention; and

- Figures 3 and 5 are side sectional views of other antennas according to the invention.

FIG. 1 shows an antenna 1 according to the invention installed at the top of a mast 2 (or of any other building) and connected by means of cables 3 to a base station 4.

In the example of FIG. 1, the antenna 1, shown in more detail in FIG. 2, is of the omnidirectional type, and makes it possible to communicate with mobile radio terminals according to three distinct frequency bands. As for example, it can be the bands at 900 MHz from GSM, at 1800 MHz from DCS and at 2000 MHz from UMTS. In this case, the base station 4 in fact gathers, functionally, three base stations corresponding to the three types of network, and three coaxial cables (feeders) connect these base stations to respective primary sources 6A, 6B, 6C of the antenna 1.

In the example shown in FIG. 2, each of the primary sources 6A-6C is a dipole tuned to a central frequency of the frequency band associated with this source. Each dipole is conventionally connected to its feeder (not shown in Figure 2) which feeds it independently of the other dipoles.

The three dipoles 6A-6C of the antenna of FIG. 2 are aligned on an X axis, and surrounded by a focusing structure having a symmetry of revolution around the X axis.

This focusing structure comprises a central medium having, with respect to radio waves, a relatively high characteristic impedance Z _c1 . When magnetic materials (μ-, = 1) are not used, this central medium will simply be chosen to have a dielectric permittivity ε- _j close to 1, so that Z _c1 ≈ Z _c0 = 120π.

This medium with high impedance occupies around each dipole 6A, 6B, 6C a cylindrical region 7A, 7B, 7C aligned and centered on this dipole. The axial height of each of these regions 7A-7C is of the order of the wavelength radiated by the corresponding dipole 6A-6C. Its radius d-, (indicated only for region 7A in FIG. 2) is of the form λ _1. (2p ₁ -1) / 4, where p-, is a positive integer preferably equal to 1, and λ -, denotes the wavelength radiated by the dipole 6A, 6B, 6C in the medium of impedance Z _c1 . The wavelength λ ₁ is given by λ- = λ _Q. jε ^ .μ ^, the wavelength λ ₀ being that radiated in a vacuum by the source 6A, 6B, 6C.

The central medium with high impedance 7A, 7B, 7C is surrounded by a focusing layer 8A, 8B, 8C formed by a medium having a relatively low characteristic impedance Z _c2 . When magnetic materials are not used (μ ₂ = 1), we choose for the focusing layer 8A,

8B, 8C a dielectric material with ε ₂ »1.

To the right of each source 6A, 6B, 6C, the thickness e ₂ of the layer of focusing 8A, 8B, 8C is taken equal to λ _2. (2p ₂ -1) / 4, where p ₂ is a positive integer preferably equal to 1, and λ = λ ₀ - ^ε 2 V-2 ^is ' ^at length wave radiated by the corresponding source 6A, 6B, 6C in the medium with low impedance.

The medium with high impedance Z _c1 used in the antenna 1 can be air. It can also be made using a honeycomb or foam material, the dielectric permittivity of which decreases with density (see "Radome Engineering Handbook, Design and Principles", JD WALTON Jr., Editions Marcel Dekker Inc., New York, 1970). Such a material can be produced from resins or polymers, for example of the polyester, epoxy, phenolic polyimide or polyurethane type.

For the low impedance focusing layers Z _c2 , it is possible in particular to use organic materials such as a polyester (ε _r from 4 to 5), an epoxy (ε _r ≈ 4) or a polyimide (ε _r = 3.5 ).

If the cost of the antenna is not the most critical factor, it is still possible to use materials having a very high permittivity, in particular inorganic compounds as used in radomes intended for high speeds and high temperatures, for example AI ₂ O ₃

(ε _r ≈ 9) or TiO ₂ (ε _r ≈ 100). Such materials can be diffused in a ceramic support matrix, for example made of silica, making it possible to adjust the value of ε _r

For cost and / or convenience reasons, it may be wise to use composite dielectrics instead of natural dielectrics to obtain desired values for the parameters ε _r and μ _r .

The term “natural dielectric” is understood here to mean a pure dielectric compound or a mixture on a microscopic scale of pure dielectric compounds. For example, polystyrene (ε _r = 2.5) or lead glass (ε _r = 7).

A composite dielectric is a macroscopic assembly of discrete metallic or dielectric particles, regularly arranged according to the three dimensions of space and in various forms: spheres, discs, bands, rods or wires. The assembly is held by a support: the particles are for example coated in a homogeneous dielectric medium, or arranged on dielectric plates. The support index is, in each case little different from 1. If the particle dimensions and the distance Inter-particles are weak compared to the wavelength, the behavior of these assemblies is identical to that of a natural dielectric. On the other hand the weight can be very reduced and the value of the dielectric constant can be adjusted quite finely. The value of ε _r for such an artificial dielectric is determined on a sample or by approximate formulas. For example, an arrangement made up of N metallic spheres of radius a per unit of volume leads to a dielectric constant of value: ε _r = 1 + 4πNa ³ . It is thus possible to obtain an ε _r ranging from 1 to 9. For mediums with high impedance Z _c1 , it is possible to similarly adjust the parameter μ _r and to obtain inexpensive weakly magnetic composite materials and low loss with an appropriate concentration of iron particles in a plastic or resin support.

The assembly of the focusing structure is for example carried out by molding, after having positioned the sources 6A-6C and their feeders. If the mechanical strength of one or other of the dielectric media requires it, it can be reinforced, for example with glass fibers. It is also possible to use support, conditioning or protection elements which do not disturb the electromagnetic behavior of the assembly. The focusing structure can also be produced in a modular fashion.

The largest dimension of the antenna 1 in FIG. 2 is its axial height which, in the example considered, can remain of the order of 50 cm. The multi-frequency antenna therefore achieves the objective of being very compact. Each of the dipoles 6A, 6B, 6C has an omnidirectional radiation diagram, with a set of focusing directions A, B, C contained in the equatorial plane of the dipole. The aforementioned resonance phenomenon increases the focusing of the waves emitted by the dipoles 6A-6C in these directions AC (focusing in elevation). The gain in amplitude provided by the composite focusing structure is given by 2.Z _c1 / Z _c2 . The gain in power g, expressed in dB, is given by g = 20.log ₁₀ (2.Z _c1 / Z _c2 ). We see that we easily get focusing gains of several decibels.

This gain can be increased by adding focusing layers, alternately of high and low impedance. The antenna 11 shown in Figure 3 has a generally planar configuration. The medium 17A, 17B, 17C with high impedance containing the dipoles (or other primary sources) 16A, 16B, 16C is deposited on a conductive ground plane 15. This medium 17A, 17B, 17C forms at the level of each source 16A, 16B , 16C a layer of thickness λ-,. (2q-1) / 2, λ-, being the wavelength radiated in the medium by the source in question, and q a positive integer advantageously equal to 1. The distance d-, between the source 16A, 16B, 16C and the interface with the first low-impedance focusing layer 18A, 18B, 18C is of the form λ _1. (2p ₁ -1) / 4. The thickness e of the (it) -th focusing layer (i> 2) is of the form λ _j . (2p _r 1) / 4. The successive focusing layers (18A, 19A, 20A), (18B, 19B, 20B), (18C, 19C, 20C) are alternately at low impedance and at high impedance, that is to say that for each odd integer i, the i-th focusing layer is formed by a medium whose characteristic impedance Z _c2 is lower than that Z _c1 of the medium located on either side of this i-th layer.

The antenna 11 according to FIG. 3 can be installed for example on a wall in order to radiate in a directive manner (directions A-C) towards an area to be covered by the base station.

FIG. 4 schematically illustrates a multisectoral antenna produced according to the invention. The geometry of the focusing structure has symmetry of revolution about the X axis, along which three primary sources 26A, 26B, 26C are aligned. Each of these primary sources is for example made up of a square conductive pattern (“patch”) formed on a dielectric substrate (microstrip technology). This type of source has a directivity in azimuth and in elevation, in a direction A, B, C perpendicular to the substrate. The focusing structure with cylindrical geometry makes it possible to increase the focusing in site and therefore the gain of the antenna 21. To limit the size of the sources 26A-26C at the heart of the focusing structure, they can be produced on a substrate at high ε _r . In the example in Figure 4, the three primary directive sources

26A-26C are tuned to the same frequency, and they are arranged on the X axis so that their focusing directions A-C are radial directions oriented at 120 ° from each other. The antenna is therefore trisectoral.

The central medium with high impedance 27 and the focusing layer 28 (and possibly the following layers, not shown) have dimensions fixed as indicated above, taking into account the wavelength radiated by the sources 26A-26C.

Note that it is possible to add to the primary sources 26A-26C forming a multisectoral antenna according to FIG. 4 an omnidirectional source such as a dipole, to form a mixed antenna.

The antenna 31 shown in FIG. 5 has a general configuration similar to that of FIG. 3, with a single focusing layer at low impedance 38A, 38B, 38C beyond the mediums 37A, 37B, 37C at high impedance containing the dipoles 36A, 36B, 36C. The various media 37A-C, 38A-C meet the spatial conditions of resonance previously considered. The interface between the successive media is inclined relative to the ground plane 35 and to the primary sources 36A-C, so that the phenomenon of refraction of the waves inclines the directions of focusing A-C, downwards in the example drawn. This makes it possible to adapt the antenna radiation pattern as needed.

In an alternative embodiment based on the same principle, the interfaces between dielectric layers are parallel to the ground plane, and it is the dipoles which are inclined. Naturally, it is also possible to similarly tilt the focusing directions in the case of an antenna with symmetry of revolution of the kind of FIG. 2 or 4, which then takes a conical rather than cylindrical shape.

An antenna according to the invention can be produced with various types of primary sources (simple or crossed dipoles, slots, microstrip patterns), each arranged outside the emission lobes of the others in order to ensure their electromagnetic decoupling.

In the case of a multisectoral antenna, the primary sources can be placed or shaped on a non-planar metal surface, for example a cylindrical or conical surface, which improves the front-rear ratio of the antenna. The cylinder or cone delimited by this surface has symmetry with respect to the axis of the antenna. It has for example a circular, triangular or polygonal section.

Claims

1. Radiocommunication base station antenna, comprising several primary sources (6A-6C, 16A-16C, 26A-26C, 36A-36C) independently supplied and arranged to have different radiation characteristics, the primary sources being placed in a first medium (7A-7C, 17A-17C, 27, 37A-37C) so as to be spatially decoupled, characterized in that it further comprises at least one second medium (8A-8C, 18A-18C , 28, 38A-38C) covering the first medium and having a significantly lower characteristic impedance than the first medium, and in that each primary source has at least one focusing direction (AC) perpendicular to the interface between the first and second medium, according to which the distance (d.,) between said primary source and said interface is substantially equal to λ _1. (2p ₁ -1) / 4 and the second medium has a thickness (e ₂ ) substantially equal to λ ₂ . (2p ₂ -1) / 4, where λ-, and λ ₂ desig the wavelengths radiated by said primary source in the first and second media, respectively, and p-, and p ₂ are integers.

2. An antenna according to claim 1, in which the second medium (8A-8C, 18A-18C, 28, 38A-38C) has a dielectric permittivity substantially higher than the first medium (7A-7C, 17A-17C, 27, 38A-38C).

3. An antenna according to claim 2, in which the second medium

(8A-8C, 18A-18C, 28, 38A-38C) has a composite dielectric material.

4. An antenna according to any one of the preceding claims, in which the first medium (17A-17C) is covered by a superposition of focusing layers (18A-18C, 19A-19C, 20A-20C), the first focusing layer (18A-18C), adjacent to the first medium, being formed by said second medium, in which each focusing layer is formed by a medium of thickness substantially equal to λ _j . (2p _j -1) / 4 in the direction of focusing (AC) of each of the primary sources (16A-16C), where λ _j denotes the wavelength radiated by said primary source in the medium forming said focusing layer and p _s is an integer, and in which the i- th focusing layer is formed, for each odd integer i, by a medium having a significantly lower characteristic impedance than the media located on either side of said i-th focusing layer.

5. Antenna according to claim 4, in which the i-th focusing layer is formed, for each odd integer i, by a medium having a dielectric permittivity substantially higher than the media located on either side of said i- th focusing layer.

6. Antenna according to claim 5, in which the i-th focusing layer is formed, for each odd integer i, by a medium comprising a composite dielectric material.

7. Antenna according to any one of claims 4 to 6, in which the i-th focusing layer is formed, for each even integer i, by a medium comprising iron particles in a support made of plastic or resin.

8. An antenna according to any one of the preceding claims, in which the primary sources (6A-6C, 16A-16C, 26A-26C, 36A-36C) comprise dipoles, radiating slots and / or microstrip patterns.

9. An antenna according to any one of the preceding claims, in which the primary sources (6A-6C, 16A-16C, 36A-36C) are supplied and arranged so as to radiate at different wavelengths.

10. An antenna according to any one of the preceding claims, in which the primary sources (6A-6C, 26A-26C) are placed along an axis (X) around which said media have a symmetry of revolution.

11. An antenna according to claim 10, in which the primary sources comprise dipoles (6A-6C) aligned on said axis (X).

12. An antenna according to claim 10, in which several of the primary sources (26A-26C) are supplied and arranged so as to radiate at substantially the same wavelength, and have focusing directions in elevation and in azimuth (AC) oriented in distinct radial directions with respect to said axis. 13 An antenna according to claim 12, in which the primary sources are shaped on a non-planar metallic surface

14 The antenna according to claim 13, wherein said non-planar metallic surface is cylindrical or conical, with symmetry with respect to said axis (X)

15 An antenna according to any one of the preceding claims, in which the first medium (37A-37C) and the second medium (38A-38C) have an interface inclined relative to the primary source (36A-36C)